1. Field of the Invention
The present invention relates to wireless communications. More specifically, the present invention relates to signal reception.
2. Background Information
A system for mobile wireless communications may support communications between a base station and a mobile unit even when the mobile unit is in motion. For example, a system for cellular telephony may support communications even when the mobile unit is moving at a high rate of speed, such as in an automobile or on a train.
Relative movement between a mobile unit and a base station in communication may affect the characteristics of the transmission channel between them, however. Specifically, this relative movement may give rise to a Doppler frequency shift that results in a spreading of the transmitted signal at the receiver. The Doppler effect may be illustrated graphically by comparing spectral plots of a transmitted signal at the transmitter and at the receiver. For example, a pilot signal of a code-division multiple-access (CDMA) system (before pseudonoise (PN) spreading and transmission by a base station) may have a delta-function magnitude spectrum as shown in FIG. 1A. As a result of the Doppler effect, the spectrum of the signal as received (and after PN despreading) by a mobile unit in motion may be spread as shown in the magnitude spectrum of FIG. 1B. The bandwidth of the spreading is a function of the carrier frequency and of the relative velocity, as described by the following expression:
                              f          d                =                              f            c                    ⁢                      v            c                                              (        1        )            where fd is the Doppler frequency, fc is the carrier frequency, c is the speed of light, and v is the relative velocity. Expression (1) demonstrates that the bandwidth of the Doppler spreading is dependent on (and increases with) the relative velocity. In a system for mobile wireless communications, the perceived relative velocity may be expected to change constantly. Additional discussion of the Doppler effect in wireless communications may be found in Microwave Mobile Communications, W. C. Takes, ed., 1974 (reprinted 1994 by IEEE Press, Piscataway, N.J.) and Wireless Communications, T. S. Rappaport, Prentice-Hall, 1996.
Bandwidth spreading as caused by the Doppler effect may create inaccuracies in a signal as received. Information relating to Doppler spreading of a signal may be applied within a receiver to improve reception of the spread signal (and possibly of other signals received over the same path). Unfortunately, existing methods for bandwidth estimation are too inefficient for practical application. What is needed is a method and apparatus to efficiently obtain a bandwidth estimate.